1. Field of The Invention
The invention relates to sonar systems, and more particularly to the processing of received narrowband acoustic data used for the purpose of detecting, classifying, and localizing acoustic sources of interest. Such processors are designed to normalize narrowband source data against a broadband interference background and to reduce such interference.
2. Description of The Prior Art
Sonar data processors of the type of interest here are designed to improve the observability of a target's spectral line structure (narrowband lines or "tonals" in the presence of substantial "non-white" broadband interference.
Target detection and classification is the purpose of narrowband processing, which exploits the presence of tonal and/or narrowband line structure in the target's radiated signal spectrum. In the basic form, this process is implemented by observing power spectrum time history plots of the acoustic sensor output in the frequency band of interest. This process, traditionally performed by trained observers, can be automated by the addition of an auto-detect and track processor.
For a well-behaved background, the auto-detect and track processor has little trouble detecting and tracking low level "lines". However, in the presence of strong broadband interference, line structure from a target of interest can easily be masked or dominated by an interference pattern in the broadband spectrum caused by multipath arrivals from the broadband source. Depending on the multipath arrival structure, the broadband interference pattern may be very broad (with respect to frequency) or very narrow. For the situation where this pattern is very narrow, broadband energy appears as a set of narrowband "lines" which cause severe post processing problems (overloading or false alerts in the auto-detect and track function).
Auto-detect and track processing as well as narrowband processor displays require a flat power spectrum background with a known mean or average value, hence "whitening" of the received signal power spectrum is required. "Whitening" is the maintenance of the envelope of the spectral background noise at a constant intensity.
Broadband interference, which can dominate the spectral background, is dynamic since the type of pattern observed is a function of signal levels, multipath structure, and geometry, all of which change with time. Thus sonar processors, which lack an adaptive capability are of limited value in treating this type of broadband interference.
Accordingly, an adaptive approach to the processing of sonar data is essential.